Pelagic Primary Production Research Articles

Exploring novel North Water Polynya ecosystems under climate change

Climate change is rapidly reshaping species distributions in the Arctic, which could profoundly impact ecosystem structure and function. While considerable effort has focused on projecting future species distributions, assessing the impacts of range-shifting species on recipient communities and subsequent disruptions to food webs remains largely unstudied. Here, we address this gap by combining species distribution models and ecosystem models to explore the emergence of novel ecosystems in the North Water Polynya. The North Water Polynya is an open-water area between Greenland and Canada, surrounded by sea ice and one of the world’s most productive ocean ecosystems. Using existing literature and projections from species distribution models of four marine species, we develop six plausible future ecosystem scenarios for the North Water Polynya. These scenarios include changing biomass of primary producers, changing biomass and size structure of copepods, shifting abundances of forage fish species, and the establishment of killer whales. We find that the biomass of higher trophic levels show pronounced decreases in response to the decrease in pelagic primary producers, with polar bear biomass halving compared to present conditions. Changes in the copepod size structure has the largest impact on the entire ecosystem compared to the other novel ecosystem scenarios, suggesting a strong reliance of higher trophic levels on large, lipid-rich copepods. We further show that increasing capelin with a simultaneous decrease in Arctic cod biomass causes large decreases in the biomass of marine mammals such as polar bear, beluga and ringed seal. Finally, we show the establishment of killer whales as a key novel predator could have cascading top-down effects on the North Water Polynya ecosystem. The framework presented here provides an approach for exploring the emergence of novel ecosystems and highlights how climate change could disrupt a high Arctic ecosystem.

Extreme mismatch between phytoplankton and grazers during Arctic spring blooms and consequences for the pelagic food-web

Food-web structure determines the cycling pathways and fate of new production in marine ecosystems. Herbivorous zooplankton populations are usually seasonally coupled with pelagic primary producers. Synchrony of phytoplankton blooms with reproduction, recruitment and seasonal ascent of their main grazers ensures efficient transfer of organic carbon to higher trophic levels, including commercially harvested species, especially in high-latitude systems. Changes in light, nutrient, and sea-ice dynamics due to accelerating climate change in the Arctic, however, create large uncertainties in how these systems will function in the future. To address such knowledge gaps, we surveyed the pelagic ecosystem of the Barents Sea Polar Front in May of two consecutive years (2021 and 2022) to investigate the pelagic food-web from primary producers to planktivorous fish. In both years we observed unprecedentedly high phytoplankton chlorophyll a values in open as well as ice-covered waters, much of which was invisible to satellite remote sensing. We also measured very low densities of grazing zooplankton across a wide area and extending for at least one month. This extreme mismatch resulted in low feeding by capelin, and further suggests a high potential for vertical export of carbon to the benthos rather than efficient assimilation into the pelagic food web. As the Arctic continues to warm and is characterized by thinner and more mobile sea ice, we may expect higher variability in phytoplankton bloom phenology and more frequent mismatches with grazer life-histories. This could have significant impacts on ecosystem functioning by re-directing the flow of energy through the system towards seafloor rather than to the production of commercially valuable pelagic marine resources.

Oceanic seabirds chase tropical cyclones

In late summer and autumn, the passage of intense tropical cyclones can profoundly perturb oceanic and coastal ecosystems. Direct negative effects on individuals and marine communities can be dramatic, especially in the coastal zone,1,2,3,4 but cyclones can also enhance pelagic primary and secondary production.5,6,7,8,9 However, cyclone impacts on open ocean marine life remain poorly understood. Here, we investigate their effects on the foraging movements of a wide-ranging higher predator, the Desertas petrel (Pterodroma deserta), in the mid-latitude North Atlantic during hurricane season. Contrary to previously studied pelagic seabirds in tropical and mid-latitude regions,10,11 Desertas petrels did not avoid cyclones by altering course, nor did they seek calmer conditions within the cyclone eye. Approximately one-third of petrels tracked from their breeding colony interacted with approaching cyclones. Upon encountering strong winds, the birds reduced ground speed, likely by spending less time in flight. A quarter of birds followed cyclone wakes for days and over thousands of kilometers, a behavior documented here for the first time. Within these wakes, tailwind support was higher than along alternative routes. Furthermore, at the mesoscale (hours-weeks and hundreds of kilometers), sea surface temperature dropped and surface chlorophyll sharply increased, suggesting direct effects on ocean stratification, primary production, and therefore presumably prey abundance and accessibility for surface-feeding petrels. We therefore hypothesize that cyclone wakes provide both predictably favorable wind conditions and foraging opportunities. As such, cyclones may have positive net effects on the demography of many mid-latitude pelagic seabirds and, likely, other marine top-predators.

Mercury dynamics at the base of the pelagic food web of the Gulf of Gdańsk, southern Baltic Sea

Planktonic organisms, which have direct contact with water, serve as the entry point for mercury (Hg), into the marine food web, impacting its levels in higher organisms, including fish, mammals, and humans who consume seafood. This study provides insights into the distribution and behavior of Hg within the Baltic Sea, specifically the Gulf of Gdańsk, focusing on pelagic primary producers and consumers. Phytoplankton Hg levels were primarily influenced by its concentrations in water, while Hg concentrations in zooplankton resulted from dietary exposure through suspended particulate matter and phytoplankton consumption. Hg uptake by planktonic organisms, particularly phytoplankton, was highly efficient, with Hg concentrations four orders of magnitude higher than those in the surrounding water. However, unlike biomagnification of Hg between SPM and zooplankton, biomagnification between zooplankton and phytoplankton was not apparent, likely due to the low trophic position and small size of primary consumers, high Hg elimination rates, and limited absorption.

Food webs in isolation: The food-web structure of a freshwater reservoir with armoured shores in a former coastal bay area

Many shallow coastal bays have been closed off from the sea to mitigate the risk of flooding, resulting in coastal reservoir lakes with artificial armoured shorelines. Often these enclosed ecosystems show a persistent decline in biodiversity and ecosystem services, which is likely reflected in their food-web structure. We therefore hypothesize that the food webs of coastal reservoir lakes with armoured shorelines (1) consist of relatively few species with a low food-web connectance and short food chains, and (2) are mainly fuelled by autochthonous organic matter produced in the pelagic zone. To investigate these two hypotheses, we used stable-isotope analysis to determine the food-web structure of lake Markermeer (The Netherlands), a large reservoir lake with armoured shorelines in a former coastal bay area. Contrary to expectation, connectance of the food web in lake Markermeer was comparable to other lakes, while food-chain length was in the higher range. However, the trophic links revealed that numerous macroinvertebrates and fish species in this constructed lake exhibited omnivorous feeding behaviour. Furthermore, in line with our second hypothesis, primary consumers heavily relied on pelagically derived organic matter, while benthic primary production exerted only a minor and seasonal influence on higher trophic levels. Stable-isotope values and the C:N ratio of sediment organic matter in the lake also aligned more closely with phytoplankton than with benthic primary producers. Moreover, terrestrial subsidies of organic matter were virtually absent in lake Markermeer. These findings support the notion that isolation of the lake through shore armouring and the lack of littoral habitats in combination with persistent resuspension of sediments have affected the food web. We argue that restoration initiatives should prioritize the establishment of land-water transition zones, thereby enhancing habitat diversity, benthic primary production, and the inflow of external organic matter while preserving pelagic primary production.

Benthic primary production in Young Sound, Northeast Greenland

The extreme and variable light climate of polar marine environments imposes a substantial limitation on benthic primary production and demands efficient adaptive capacities of the primary producers. This chapter reviews the composition, abundance, primary production and adaptive strategies of benthic primary producers in Young Sound. Benthic primary producers occurred in the 0-50 m depth range and the relative importance of microalgae, crustose coralline macroalgae and foliose macroalgae varied systematically with depth. On a summer day with optimal light conditions benthic primary production showed a maximum of c.
 70 mmol O2 m-2 d-1 in shallow water. Production rates declined gradually to c. 20 mmol O2 m-2 d-1at 10–20 m depth and to 2.5 mmol O2 m-2 d-1 at 30 m depth.
 Foliose macroalgae contributed markedly to primary production in shallow water but became insignificant at water depths >15 m, while benthic diatoms contributed most to primary production at intermediate water depths (5-30 m). At water depths greater than 30 m only coralline algae occurred, but their production was low because of their low abundance, low Pmax and the low ambient irradiance at those depths. All algal groups were well adapted to the ambient irradiance and could, within minutes, acclimate their photosynthetic performance to changing light conditions. The benthic primary production in Young Sound markedly surpassed the pelagic primary production down to water depths of 20 m and the results thereby underline the potential importance of benthic primary production in shallow-water Arctic ecosystems.

Benthic contribution to seasonal silica budgets in two macrotidal estuaries in North-Western France

The paper aims to build seasonal silica budgets in two macrotidal estuaries, the Elorn and Aulne estuaries of the Bay of Brest (North-Western France), based on modeling and measurements, in order to increase our understanding of the silica (Si) cycle at land-sea interfaces. A diagenetic model was developed to quantify benthic Si fluxes, e.g. aSiO2 deposition fluxes that are difficult to assess through direct measurements. Sediment cores were also seasonally sampled at six stations to provide data essential to parametrize and validate the model. Vertical profiles of porosity, burrowing depth, biodiffusive coefficients, concentrations of amorphous silica (aSiO2) and silicic acid (Si(OH)4 and the proportion of reactive aSiO2 were measured. The results show that sites sampled along the Elorn and Aulne estuaries constitute significant net Si deposition areas (1-4.5 mmol Si m-2 d-1), particularly in the upstream during winter and in midstream and downstream during summer. Year round, reprecipitation is negligible (< 3%) while burial accounts for the retention of ~ 30-80% of deposited aSiO2. In winter, burial dominates the benthic Si budget. As surface-integrated benthic Si fluxes are low compared to riverine aSiO2 fluxes, the Si export to coastal waters is high (93%) during winter. In contrast, in summer, burial accounts for 38% of river Si fluxes, and Si(OH)4 flux from the sediment is high as a result of enhanced benthic recycling and bioirrigation. Internal estuarine processes, e.g., benthic and pelagic primary production, dissolution and benthic Si fluxes, surpass river fluxes in magnitude during summer. Overall, we conclude that the Elorn and Aulne macrotidal estuaries are efficient filters of Si, retaining about 4-38% of river Si fluxes, and even 6-67% when accounting for retention in intertidal marshes, but with massive exports occurring during winter floods.

Interannual differences in sea ice regime in the north-western Barents Sea cause major changes in summer pelagic production and export mechanisms

The Barents Sea is a highly dynamic and productive marine ecosystem and a hotspot of global warming. Variability in sea ice extent is a common feature in the Barents Sea with substantial movements of the sea ice edge on short-term, seasonal to interannual time scales. Historically the northern Barents Sea (north of 75°N) has been ice-covered in winter, but recently it has become the area with most winter ice loss in the Arctic, and year-round ice-free conditions are predicted for the second half of the 21st century. These environmental changes have significant implications for the marine ecosystem. In this study we used contrasting sea ice regimes in August 2018 and August 2019 to explore the response of phytoplankton and bacterial production, microbial abundance, and vertical carbon flux in the north-western Barents Sea (between 76°N and 83°N) to the variability of sea ice. While the study area was ice-free in August 2018, extensive areas north of 79°N were ice-covered in 2019. When the northern parts of the transect were still ice covered, diatoms and other larger phytoplankton were dominant and highest abundances were observed following the receding ice edge. In contrast, under ice-free conditions in 2018, the pelagic ecosystem resembled a post-bloom stage of the seasonal succession with higher abundance of small phytoplankton and heterotrophic protists and low vertical flux throughout the water column. While phytoplankton biomass, bacterial production and downward vertical flux of particulate organic carbon in the upper 60 m were on average higher in 2019, primary production and carbon export below the euphotic layer were comparable between both years. However, overall highest primary production, bacterial production and abundance of both photosynthetic and heterotrophic microorganisms were observed in surface waters (upper 30 m) in 2019, connected to the retreating ice edge, where also vertical particle flux was higher and characterized by a strong attenuation curve. The results clearly demonstrate that differences in ice cover affect the phenology of pelagic primary production and associated biological processes in the Barents Sea.

Dependency of Arctic zooplankton on pelagic food sources: New insights from fatty acid and stable isotope analyses

AbstractGlobal warming causes dramatic environmental change to Arctic ecosystems. While pelagic primary production is initiated earlier and its intensity can be increased due to earlier ice melt and extended open‐water periods, sea‐ice primary production is progressively confined on a spatio‐temporal scale, leading to unknown consequences for the ice‐associated (sympagic) food web. Understanding ecological responses to changes in the availability and composition of pelagic and sympagic food sources is crucial to determine potential changes of food‐web structure and functioning in Arctic marine communities under increasingly ice‐free conditions. Focus was placed on the importance of suspended particulate organic matter vs. sympagic organic matter for 12 zooplankton species with different feeding modes covering five taxonomic groups (copepods, krill, amphipods, chaetognaths, and appendicularians) at two ice‐covered, but environmentally different, stations in the north‐western Barents Sea in August 2019. Contributions of diatom‐ and flagellate‐associated fatty acids (FAs) to total lipid content and carbon stable isotopic compositions of these FAs were used to discriminate food sources and trace flows of organic matter in marine food webs. Combination of proportional contributions of FA markers with FA isotopic composition indicated that consumers mostly relied, directly (herbivorous species), or indirectly (omnivorous and carnivorous species), on pelagic diatoms and flagellates, independently of environmental conditions at the sampling locations, trophic position, and feeding mode. Differences were nevertheless observed between species. Contrary to other studies demonstrating a high importance of sympagic organic matter for food‐web processes, our results highlight the complexity and variability of trophic structures and dependencies in different Arctic food webs.

Opportunistic consumption of marine pelagic, terrestrial, and chemosynthetic organic matter by macrofauna on the Arctic shelf: a stable isotope approach.

Macrofauna can contribute substantially to the organic matter cycling on the seafloor, yet the role of terrestrial and chemosynthetic organic matter in the diets of microphagous (deposit and suspension) feeders is poorly understood. In the present study, we used stable isotopes of carbon and nitrogen to test the hypothesis that the terrestrial organic matter supplied with river runoff and local chemosynthetic production at methane seeps might be important organic matter sources for macrofaunal consumers on the Laptev Sea shelf. We sampled locations from three habitats with the presumed differences in organic matter supply: "Delta" with terrestrial inputs from the Lena River, "Background" on the northern part of the shelf with pelagic production as the main organic matter source, and "Seep" in the areas with detected methane seepage, where chemosynthetic production might be available. Macrobenthic communities inhabiting each of the habitats were characterized by a distinct isotopic niche, mostly in terms of δ13C values, directly reflecting differences in the origin of organic matter supply, while δ15N values mostly reflected the feeding group (surface deposit/suspension feeders, subsurface deposit feeders, and carnivores). We conclude that both terrestrial and chemosynthetic organic matter sources might be substitutes for pelagic primary production in the benthic food webs on the largely oligotrophic Laptev Sea shelf. Furthermore, species-specific differences in the isotopic niches of species belonging to the same feeding group are discussed, as well as the isotopic niches of the symbiotrophic tubeworm Oligobrachia sp. and the rissoid gastropod Frigidoalvania sp., which are exclusively associated with methane seeps.

Quantifying pelagic primary production and respiration via an automated in situ incubation system

AbstractPelagic photosynthesis and respiration serve critical roles in controlling the dissolved oxygen (DO) concentration in seawater. The consumption and production via pelagic primary production are of particular importance in the surface ocean and in freshwater ecosystems where photosynthetically active radiation is abundant. However, the dynamic nature and large degree of heterogeneity in these ecosystems pose substantial challenges for providing accurate estimates of marine primary production and metabolic state. The resulting lack of higher‐resolution data in these systems hinders efforts in scaling and including primary production in predictive models. To bridge the gap, we developed and validated a novel automated water incubator that measures in situ rates of photosynthesis and respiration. The automated water incubation system uses commercially available optodes and microcontrollers to record continuous measurements of DO within a closed chamber at desired intervals. With fast response optodes, the incubation system produced measurements of photosynthesis and respiration with an hourly resolution, resolving diel signals in the water column. The high temporal resolution of the time series also enabled the development of Monte Carlo simulation as a new data analysis technique to calculate DO fluxes, with improved performance in noisy time series. Deployment of the incubator was conducted near Ucantena Island, Massachusetts, U.S.A. The data captured diel fluctuations in metabolic fluxes with an hourly resolution, allowed for a more accurate correlation between oxygen cycling and environmental conditions, and provided improved characterization of the pelagic metabolic state.

A Decade of Death and Other Dynamics: Deepening Perspectives on the Diversity and Distribution of Sea Stars and Wasting.

AbstractMass mortality events provide valuable insight into biological extremes and also ecological interactions more generally. The sea star wasting epidemic that began in 2013 catalyzed study of the microbiome, genetics, population dynamics, and community ecology of several high-profile species inhabiting the northeastern Pacific but exposed a dearth of information on the diversity, distributions, and impacts of sea star wasting for many lesser-known sea stars and a need for integration across scales. Here, we combine datasets from single-site to coast-wide studies, across time lines from weeks to decades, for 65 species. We evaluated the impacts of abiotic characteristics hypothetically associated with sea star wasting (sea surface temperature, pelagic primary productivity, upwelling wind forcing, wave exposure, freshwater runoff) and species characteristics (depth distribution, developmental mode, diet, habitat, reproductive period). We find that the 2010s sea star wasting outbreak clearly affected a little over a dozen species, primarily intertidal and shallow subtidal taxa, causing instantaneous wasting prevalence rates of 5%-80%. Despite the collapse of some populations within weeks, environmental and species variation protracted the outbreak, which lasted 2-3 years from onset until declining to chronic background rates of ∼2% sea star wasting prevalence. Recruitment began immediately in many species, and in general, sea star assemblages trended toward recovery; however, recovery was heterogeneous, and a marine heatwave in 2019 raised concerns of a second decline. The abiotic stressors most associated with the 2010s sea star wasting outbreak were elevated sea surface temperature and low wave exposure, as well as freshwater discharge in the north. However, detailed data speaking directly to the biological, ecological, and environmental cause(s) and consequences of the sea star wasting outbreak remain limited in scope, unavoidably retrospective, and perhaps always indeterminate. Redressing this shortfall for the future will require a broad spectrum of monitoring studies not less than the taxonomically broad cross-scale framework we have modeled in this synthesis.

Variation in fatty acid content among benthic invertebrates in a seasonally driven system

AbstractAt temperate latitudes where seasonal changing environmental conditions strongly affect the magnitude, duration and species composition of pelagic primary production, macrobenthic organisms living below the photic zone rely on the sedimentation of organic matter as their primary energy source. The succession from nutritious spring blooms to summer cyanobacteria is assumed to reduce food quality for benthic primary consumers and their fatty acid (FA) profiles. In contrast, we find low seasonal variability in FA content of five benthic macroinvertebrates spanning two trophic levels in the Baltic Sea, a system with high seasonal variation in phytoplankton species composition. However, levels of the major FA groups vary greatly between benthic species. The results suggest that benthic macroinvertebrates have evolved FA metabolism adapted to degraded sedimenting material. Moreover, our study shows that species composition of benthic macrofauna rather than seasonal changing conditions affect availability of essential nutrients to higher trophic levels.

Gross pelagic primary production in the Ems-Dollard estuary

Anthropogenic changes to estuaries, usually highly productive systems, may result in increased turbidity levels, suppress pelagic primary production and negatively affect ecosystem functions. The Ems-Dollard estuary, enclosed between the Netherlands and Germany, is such an estuary with a long history of human induced changes. From the mid-19th century until 1992, discharges of potato starch and straw-board industry waste water, with high contents of organic matter, negatively affected oxygen concentrations and increased nutrient levels of the system. The growing awareness of the deteriorating effects of these discharges on the ecosystem invoked a large research program, BOEDE, between 1976 and 1980. The aim of the current paper was to establish the recent gross pelagic primary production, to compare the results with the late seventies data and to relate possible changes to environmental variables such as suspended matter and nutrients. To this end, six stations from the North Sea into the inner area of the Dollard were visited 39 times in 2012–2013. During each sampling cruise, a so-called PocketBox, containing a number of sensors, provided a high spatial data resolution between stations. At each station, samples were collected for carbon fixation incubations using 14C. Average chlorophyll-a concentrations as reported in this study were lower compared to 1978–1980 for the seaward stations, and higher for the most inward station in 2012 and almost similar in 2013. Light attenuation coefficients in the outer areas were lower in 2012–2013 (1.22 m−1) compared to the values reported in the period 1976–1980 (1.59 m−1), indicating somewhat less turbid conditions. For the two other stations, light attenuation coefficients in the present study were similar to the late seventies' values, for the station near the entrance of the Ems River in the Ems-Dollard area, the coefficient was substantially higher. Pelagic primary production in the 2012–2013 period was about 120 g C m−2 y−1, mainly determined by the most seaward areas. The largest differences in gross primary production in the current study compared to the 1976–1980 period were found at the seaward stations where production in the late seventies was about 60% higher. In the Dollard-area, production hardly changed since the late seventies, but also stations near the Ems-river entrance into the Ems-Dollard area showed a clear decrease. Changes at the inward stations are to be contributed to an increased turbidity; the decrease at the outer areas to a decreased nutrient level, especially dissolved phosphorus and silicate, but possibly also nitrogen.

Spatial and temporal variations in the isotope hydrobiogeochemistry of a managed river draining towards the southern Baltic Sea

The flow path of a river draining a lowland into the southern Baltic Sea, the Warnow River, was investigated to evaluate its freshwater composition and potential to be a source of dissolved substances for regional coastal waters. A spatial study was conducted in April 2019 to follow the variations from the source to the estuary. A temporal study of the composition, for five years (2017–2021), was carried out at a site just before the river reaches the estuary. Surface water was sampled to analyse major and trace elements, stable (H, C, O, S), and short-lived (Ra) isotopes. The results show that the composition of the Warnow River along the flow path is controlled by a complex interplay between in-situ processes, exchange with the atmosphere, diffuse groundwater, and surface water inlets. On a temporal scale, pH, nutrient, and redox-sensitive trace element concentrations are strongly impacted by pelagic primary production in spring. During summer and autumn, influences occurred due to benthic microbial activity, associated diffusive release from soils/sediments, and surface water inlets. Throughout the investigation period, the Warnow River was a source of isotopically light CO2 to the atmosphere and DIC to the estuarine waters. The delivered DIC concentrations seemed to vary with season due to changes in biological pelagic and benthic activities. DOC was derived from a mixture of dominated C3 organic sources and potentially from fertilizers. From concentration-discharge relationships, examples of dilution, mobilization, and chemostasis trends were found. Discharge-controlled seasonal trends were superimposed by system internal processes and the hydrological consequences of the river management. Our analysis thus provides new insights into the controls on the variations of water and solutes in a managed river at the land-sea interface as part of the regional hydrological cycle of a lowland catchment-coastal water system.

Benthic nitrogen cycling in the deep ocean of the Kuroshio Extension region

Benthic nitrogen cycling, including nitrification, N-loss, and other nitrogen transformations, plays a crucial role in the marine nitrogen budget. However, studies on benthic nitrogen cycling mainly focus on marginal seas, while attention to the deep ocean, which occupies the largest area of the seafloor, is severely lacking. In this study, we investigate the benthic nitrogen cycling in the Kuroshio Extension region (KE) of the northwest Pacific Ocean at water depths greater than 5,000 m through 15N enrichment slurry incubation and pore-water dissolved oxygen and inorganic nitrogen profiles. The slurry incubation indicates nitrification is the predominant process in benthic nitrogen cycling. The potential nitrification rates are nearly an order of magnitude higher than dissimilatory nitrate reduction. Nitrification and total N-loss flux estimated from pore-water nitrate and ammonium profiles are 6–42 and 5–30 μmol N m−2 d−1, respectively. Generally, anammox is the predominant N-loss process in KE sediment. The temperature gradient experiment indicates that the optimum temperature for anammox and denitrification is 13 and 41°C, respectively, partially explaining anammox as the dominant process for deep-ocean benthic N-loss. Both the low concentration of ammonium in pore-water and the discrepant results between anoxic incubation amended with 15NO3− and 15NH4++14NO3− suggest that ammonium is another limiting factor for benthic anammox. N-loss activity gradually declines with the distance from the Oyashio–Kuroshio transition zone. However, nitrification has the opposite trend roughly. This reveals that the sediment in KE transfers from nitrate sink to source from north to south. This trend is mainly caused by the variation of primary production and the supplement of active organic matter, which is the energy source for microbes and the potential source for ammonium through remineralization. Overall, our results highlight temperature and ammonium as two limiting factors for deep-ocean benthic N-loss and also exhibit a tight coupling relationship between pelagic primary production and the benthic nitrogen cycle in KE.

Benthic-Pelagic Coupling of Marine Primary Producers Under Different Natural and Human-Induced Pressures’ Regimes

Marine primary producers are highly sensitive to environmental deterioration caused by natural and human-induced stressors. Following the Water Framework Directive and the Marine Strategy Framework Directive requirements, the importance of using the different primary producers of the coastal marine ecosystem (pelagic: phytoplankton and benthic: macroalgae and angiosperms) as appropriate tools for an integrated assessment of the ecological status of the coastal environment has been recognized. However, the processes by which water column characteristics and phytobenthic indicators are linked have not been systematically studied. Based on a large dataset from three Mediterranean sub-basins (Adriatic, Ionian and Aegean Seas) with different trophic conditions, this study aims to explore the coupled responses of benthic and pelagic primary producers to eutrophication pressures on a large scale, focusing on the structural and functional traits of benthic macroalgal and angiosperm communities, and to investigate the key drivers among the different eutrophication-related pelagic indicators (such as nutrient and Chl-a concentrations, water transparency, etc.) that can force the benthic system indicators to low ecological quality levels. In addition to the effects of high nutrient loading on phytoplankton biomass, our results also show that increased nutrient concentrations in seawater have a similar effect on macroalgal communities. Indeed, increasing nutrient concentrations lead to increased coverage of opportunistic macroalgal species at the expense of canopy-forming species. Most structural traits ofPosidonia oceanica(expressed either as individual metrics: shoot density, lower limit depth and lower limit type, or in the context of PREI index) show opposite trends to increasing levels of pressure indicators such as ammonium, nitrate, phosphate, Chl-a and light attenuation. Furthermore, our results highlight the regulating effect of light availability on the ecological status of seagrass meadows (Posidonia oceanicaandCymodocea nodosa). Increasing leaf length values ofC. nodosaare closely associated with higher turbidity values linked to higher phytoplankton biomass (expressed as Chl-a). Overall, the coupling of pelagic and benthic primary producers showed consistent patterns across trophic gradients at the subregional scale.

Drivers for primary producers’ dynamics: New insights on annual benthos pelagos monitoring in anthropised freshwater marshes (Charente-Maritime, France)

Wetlands, especially marshes, support many services such as carbon catchment control or water purification led by primary producers such as phytoplankton and microphytobenthos (PB). The impact of the sedimentary compartment, as source and sink of essential nutrients for the water column, is often neglected in the study of their dynamics and water purification capacity of the systems. This work compared monthly (between February 2020 and April 2021) the benthic and pelagic primary producers’ dynamics in two anthropised freshwater marshes (Marans and Genouillé), with the simultaneous follow-up of physico-chemical parameters of the water column and nutrient fluxes at the sediment-water (SWI) interface. It was suggested a strong contribution of phytoplankton (pumping) and the benthic compartment (denitrification) to the water purification of these two nitrates (NO3−)-rich marshes. Total phytoplankton production fluctuated between ∼5 (winter) and 1500 mg C m−3 d−1 (fall) at Marans and between 40 (winter) and ∼750 mg C m−3 d−1 (spring) at Genouillé. At Marans, soluble reactive phosphorus (SRP) benthic effluxes (-2.101 to -6.102 µmol m−2 d−1 in fall and summer, respectively) coincided with phytoplankton bloom periods. These effluxes were inhibited by NO3− penetration in the sediment (0 to 5.104 µmol m−2 d−1), by inhibiting iron respiration. At Genouillé, inhibition of SRP effluxes depended on denitrification rate and on P stocks in the sediment, where slight SRP effluxes (-101 µmol m−2 d−1) could have co-occurred with slight NO3− influxes (5.102 µmol m−2 d−1) in spring. The presence of PB (between 10–60 and 40–120 mg gsed−1 at Marans and Genouillé, respectively), suggested a strong contribution of the benthic compartment to the total primary production (benthic and pelagic through resuspension processes) in these environments. This work encourages to consider the benthos and the pelagos as a unicum to provide better sustainable management of such systems and limit eutrophication risks in coastal areas.

Evaluation of the Beaufort Sea shelf structure and function in support of the Tarium Niryutait Marine Protected Area

Arctic ecosystems are at risk to climate impacts, challenging existing conservation measures such as protected areas. This study aims to describe the ecological dynamics of the Canadian Beaufort Sea Shelf (BSS) ecosystem and the Tarium Niryutait Marine Protected Area (TNMPA) under historical changes in sea surface temperature and sea ice extent. Using Ecopath with Ecosim, we compared the status of the BSS between two time periods, 1970–1974 and 2008–2012, and against four ecosystem models (Eastern Chukchi Sea, Barents Sea, Eastern Bering Sea, Gulf of Alaska) to inform the relative long-term health and status of Arctic marine ecosystems. We find that relative to the comparable ecosystems, the BSS had a greater proportion of biomass from pelagic primary and secondary producers, and limited production from higher trophic levels. Estimates of trophic structure indices for the BSS indicate temporal ecosystem stability, and no loss in diversity. While beluga whales are a focus of the TNMPA management plan, they are not considered a key component of the modeled food web. Rather, Arctic and polar cods (main beluga prey group), arthropods, large copepods, micro-zooplankton, and herring and smelt, were identified as keystone species and warrant attention as proxies for both beluga whales and ecosystem health.

Multiple-stressor exposure of aquatic food webs: Nitrate and warming modulate the effect of pesticides

Shallow lakes provide essential ecological and environmental services but are exposed to multiple stressors, including agricultural runoff (ARO) and climate warming, which may act on different target receptors disrupting their normal functioning. We performed a microcosm experiment to determine the individual and combined effects of three stressors—pesticides, nitrate and climate warming—on two trophic levels representative of communities found in shallow lakes. We used three submerged macrophyte species (Myriophyllum spicatum, Potamogeton perfoliatus, Elodea nuttallii), eight benthic or pelagic microalgal species and three primary consumer species (Daphnia magna, Lymnaea stagnalis, Dreissena polymorpha) with different feeding preferences for benthic and pelagic primary producers. Eight different treatments consisted of a control, only nitrate, a pesticide cocktail, and a combination of nitrate and pesticides representing ARO, each replicated at ambient temperature and +3.5°C, mimicking climate warming. Pesticides negatively affected all functional groups except phytoplankton, which increased. Warming and nitrate modified these effects. Strong but opposite pesticide and warming effects on Myriophyllum drove the response of the total macrophyte biomass. Nitrate significantly suppressed Myriophyllum final biomass, but not overall macrophyte and microalgal biomass. Nitrate and pesticides in combination caused a macrophyte decline, and the system tipped towards phytoplankton dominance. Strong synergistic or even reversed stressor interaction effects were observed for macrophytes or periphyton. We emphasize the need for more complex community- and ecosystem-level studies incorporating multiple stressor scenarios to define safe operating spaces.